The present invention relates to a plating layer for a sliding portion and a method for forming the same.
It is known that some treatments are applied to the trochoidal surface of the rotor housing forming the combustion chamber of the rotary piston engine. The apex seals of the engine slide on this trochoidal surface of the housing. Herein, the chromium molybdenum (CrMo) alloy plating layer with hardness of about Hv1000 is formed on this trochoidal surface for the superior wear resistance, and the first reverse current treatment is applied soon after the plating, and then the second reverse current treatment is applied after the honing in order to form micro-cracks at the surface for the improvement of the lubricating property by enhancing lubricating-oil retaining property (see U.S. Pat. No. 4,729,729).
It is also known that the composite chromium plating layer is formed on the cylinder liner of the reciprocating engine by applying the direct/reverse current treatments repeatedly in the chromium bath containing hard particulates and then by applying the honing (see Japanese Patent Laid-Open Publication No. 10-148155). Accordingly, the inner surface of the cylinder liner is formed with the maximum roughness of 3 μm, the porosity of 5% or less and the average depth of 0.01 mm or less.
Further, another treatment for the sliding member, such as piston rings of the internal combustion engine, have been proposed (see EP Patent Application No. 1253220 A1). Herein, the sliding member has a plurality of hard Cr plating layers laminated thereon, wherein the individual hard Cr plating layer has fine cracks formed at the surface and the fine cracks form fine voids being independent in the direction of film formation. The plating layer surface has the porosity of 0.5 to 4.5% and the number of micro-cracks of 50 to 1200 per 10 mm length.
Meanwhile, in order to obtain an appropriate hardness, the Cr plating, such as the chromium molybdenum alloy plating or the hard chromium plating, needs to be conducted with treatment of the low temperature and low current density. For example, the direct current treatment of 6.5 hours is required for the rotor housing to form the plating layer of 150 μm film thickness. This is, however, a factor of increasing parts cost that would deteriorate the productivity. Also, the above-described treatment with the second reverse current treatment after the honing causes complicate treatments that would be disadvantageous in the productivity and the costs reduction. Further, this would cause an improperly rough finish eventually. Because the final roughness of the surface to be treated is affected by the previous treatments of the first current treatment and the honing as well. Thus, this treatment also has a problem that the final product would have a poor quality.
Also, although sulfuric acid in the normal hard chromium plating (Sargent bath) functions as a catalyst to deposit Cr in the plating bath and the micro-cracks can be formed at the surface by applying the reverse current treatment to the deposited plating layer, the width of the opening of cracks is 5 μm or more and the crack's number per a unit length is 100/cm or less.
Although the width of the opening of cracks could be reduced by adjusting conditions of the reverse current treatment (reducing the current density or shortening the treatment time) and thereby reducing the amount of removed plating, this should be difficult in fact. Because, the width of the opening of cracks is generally influenced by the inner stress of the plating layer, and the plating layer formed in the conventional Sargent bath has the high internal stress. Therefore, once the cracks occur, the width of the opening would remain 5 μm or more. Meanwhile, the converse adjustment of conditions of the reverse current treatment (increasing the current density or prolonging the treatment time) to increase the amount of removed plating could promote the width of the opening of cracks. However, since the number of cracks depends on the internal stress of the plating layer, there is actually its limit of 100/cm.
The cracks with the wide opening width and the small number could not retain the lubricating oil properly on the surface of the plating layer because the lubricating oil easily goes into the cracks, and thereby the seizure tends to occur and the fuel consumption of the engine would increase due to an increased friction loss at the sliding surface.
Meanwhile, although the above-described publication discloses the plating layer forming to provide many cracks, repeated treatments of the direct and reverse current treatments are required and the adjustment of current density would be difficult actually.